Along with nitrogen (N) and potassium (K), phosphorus (P) is one of the big three nutrients required for plant growth. Phosphorus is used by plants in photosynthesis, energy transfers and nutrient transport within the plant – all vital functions for good plant growth and maximum yields. Phosphorus is applied through organic sources – manures primarily – or inorganic sources derived from processed rock phosphate. Phosphorus deficiencies are usually expressed as a purplish color on the leaves (Photo 1) which goes away after the weather and soil warms.

Phosphorus has its greatest effect on young seedlings in cold soils – less than 55 degrees Fahrenheit – containing less than 180 pounds per acre available P. Many Michigan agricultural soils are well above this level due to past routine P applications. Indiscriminate P use in agriculture, landscape and detergents increases P levels in lakes and streams, contributing to rampant algae growth. Phosphorus levels have been controlled in detergents since the 1960s, and lawn fertilizers are now available with no or low P levels. Some early planted vegetable crops will show a positive response to P even in soils with more than 180 pounds P per acre. However, roots in cold soils grow slowly and do not encounter enough P, so placing P close to young roots in a liquid “starter fertilizer” can help.

Phosphorus use in agriculture is a continual concern and growers should review how they use it from an environmental, as well as an economic, standpoint. This is especially true for vegetable growers using raised, plastic-mulched beds. Plastic mulch raises soil temperature 5 F to 10 F or more, depending on the plastic and time of year. This temperature increase can be enough to allow roots to grow at a fast enough rate to supply the plant with all the P it needs. This would especially be true for anything planted in May with the increasing angle of the sun.

MSU Extension is making a concerted effort to help vegetable growers reduce pesticide and nutrient use. If this article helped you reduce P, please let us now by contacting Ron Goldy or Emily Sneller using the e-mail addresses below.